Assymetric radio frequency magnetic line array

ABSTRACT

An apparatus comprises a radio frequency magnetic field unit to generate a desired magnetic field. In one embodiment, the radio frequency magnetic field unit includes a first aperture that is substantially unobstructed and a second aperture contiguous to the first aperture. In an alternative embodiment, the radio frequency magnetic field unit includes a first side aperture, a second side aperture and one or more end apertures. In one embodiment of a method, a current element is removed from a radio frequency magnetic field unit to form a magnetic field unit having an aperture. In an alternative embodiment, two current elements located opposite from one another in a radio frequency magnetic field unit are removed to form a magnetic filed unit having a first side aperture and a second side aperture.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/637,261, filed Aug. 8, 2003, which is a continuation of U.S. patentapplication Ser. No. 09/919,479, filed Jul. 31, 2001, which claimspriority under 35 U.S.C. 119(e) from U.S. Provisional Application Ser.No. 60/222,144, filed Jul. 31, 2000, which applications are incorporatedby reference herein.

FIELD

This invention relates to radio frequency magnetic field units suitablefor use in connection with an imaging and/or spectroscopy system.

BACKGROUND

Radio frequency magnetic field units, such as volume coils, are used inconnection with imaging and/or spectroscopy systems, such as but notlimited to magnetic resonance imaging systems, nuclear magneticresonance imaging systems, functional magnetic resonance imagingsystems, and electron spin resonance systems.

A problem with many cylindrical form volume coils is that they providelimited access to the coil volume. These cylindrical form volume coilscan be accessed only through the ends of the cylinders or between theradio frequency (RF) current carrying rungs or loops. The “between therung” or lateral access is further limited when the coil is shielded. AFaraday shield on a birdcage for example, completely screens the lateralwalls of the coil cylinder with typically a copper clad, etched circuitboard material. The result is a “copper can.” Similarly, the transverseelectromagnetic (TEM) coil circuits are composed of a cylindricalsymmetrical array of conductor rungs in parallel resonance with andenclosed by a copper resonant cavity. The limited access provided by endaccess or “between the rung” access to the coil volume affects a subjectconfined to the coil volume and physicians or technicians treating orinteracting with the subject. Some subjects are claustrophobic andcannot tolerate confinement in a volume coil, while some medicalprocedures, such as brain surgery, require access to the subject duringimaging. For these and other reasons there is a need for the presentinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is an illustration of some embodiments of an apparatus includinga radio frequency magnetic field unit according to the teachings of thepresent invention;

FIG. 1B is an illustration of some embodiments of an imaging unitincluding a radio frequency magnetic field unit according to theteachings of the present invention;

FIG. 2A is an illustration of some embodiments of an alternativeembodiment of an apparatus including a radio frequency magnetic fieldunit according to the teachings of the present invention;

FIG. 2B is an illustration of some alternative embodiments of an imagingunit including a radio frequency magnetic field unit according to theteachings of the present invention;

FIG. 2C is an illustration of some embodiments of the apparatusincluding the radio frequency magnetic field unit shown in FIG. 2Aconfigured for use in a clinical setting;

FIG. 3A-3D are illustrations of some embodiments of the structure of avolume coil according to the teachings of the present invention; and

FIG. 4 is an illustration of one embodiment of a current elementsuitable for use in connection with the radio frequency magnetic fieldunits of the present invention.

FIGS. 5A, 5B, 5C, 5D compare lumped element resonant circuits totransmission line analogues.

FIGS. 6A and 6B show alternative circuit models for a tuned TEMresonator according to the present subject matter.

DESCRIPTION

In the following detailed description of the invention, reference ismade to the accompanying drawings which form a part hereof, and in whichare shown, by way of illustration, specific embodiments of the inventionwhich may be practiced. In the drawings, like numerals describesubstantially similar components throughout the several views. Theseembodiments are described in sufficient detail to enable those skilledin the art to practice the invention. Other embodiments may be utilizedand structural, logical, and electrical changes may be made withoutdeparting from the scope of the present invention. The followingdetailed description is not to be taken in a limiting sense, and thescope of the present invention is defined only by the appended claims,along with the full scope of equivalents to which such claims areentitled.

Radio frequency magnetic field units that include an aperture that issubstantially unobstructed and located in a radio frequency magneticfield unit and radio frequency magnetic field units that include sideapertures are described. When a radio frequency magnetic field unit thatincludes a first aperture that is substantially unobstructed is used inconnection with an imaging system, the medical benefits associated withthe use of an imaging system can be extended to claustrophobic subjects.When a radio frequency magnetic field unit that includes side aperturesis used in connection with an imaging system, the medical benefitsassociated with the use of an imaging system can be extended to subjectsthat have difficulty fitting into a standard radio frequency magneticfield unit. Methods for transforming a radio frequency magnetic fieldunit that lacks an aperture that is substantially unobstructed into aradio frequency magnetic field unit that has an aperture that issubstantially unobstructed, and methods for transforming a radiofrequency magnetic field unit that lacks side apertures into a radiofrequency magnetic field unit that has side apertures are alsodescribed.

In addition, including an aperture in a radio frequency magnetic filedunit, such as a coil, allows parts of the anatomy to project from thecoil (e.g. nose, or arms). This allows the rest of the coil to be muchsmaller and fit much closer to a subject. Small, close fitting coilsimprove image signal efficiency which results in images of higherresolution being acquired in less time using less power.

FIG. 1A is an illustration of some embodiments of an apparatus 100comprising a radio frequency magnetic field unit 102 according to theteachings of the present invention. The radio frequency magnetic fieldunit 102 includes a first aperture 104 and a second aperture 106.

The radio frequency magnetic field unit 102 generates a desired magneticfield 108. The desired magnetic field 108 is not limited to a magneticfield having a particular magnitude and direction. Preferably, thedesired magnetic field 108 has a magnitude and direction suitable foruse in imaging an object, such as a human head, in an imaging system,such as but not limited to a magnetic resonance imaging system, amagnetic resonance spectroscopy system, a functional magnetic resonanceimaging system, or an electron spin resonance system.

The radio frequency magnetic field unit 102 is not limited to aparticular type of radio frequency magnetic field unit. In oneembodiment, the radio frequency magnetic field unit 102 is a TEM cavityresonator. A TEM cavity resonator includes one or more current elementshaving controllable elements, such as inductors and capacitors, that arevaried to tune the transmission line resonator. In some embodiments, TEMcavity resonators include two open ends. In alternative embodiments, TEMcavity resonators include one open end and one closed end.

The radio frequency magnetic field unit 102 is not limited to aparticular number of current elements (shown in FIG. 4). Currentelements 110-115 shown in FIG. 1A are illustrations only and no attemptis being made to depict the detailed components of the current elements.In some embodiments, the radio frequency magnetic field unit 102includes current elements 110-115. The current elements 110-115 arepreferably arranged such that none of the elements 110-115 obstruct thesecond aperture 106, but the current elements 110-115 are not limited toa particular arrangement. In some embodiments, the current elements110-115 are asymmetrically arranged, physically disconnected from oneanother and reactively coupled. In alternative embodiments, the currentelements 110-115 are arranged to “enclose” a substantially cylindricalvolume. In some embodiments, each of the current elements 110-115 is aresonant current element inductively coupled to at least one othercurrent element. In alternative embodiments, each of the currentelements 110-115 is a resonant current element capacitively coupled toat least one of the current elements 110-115. When used in connectionwith an imaging system (shown in FIG. 1B), the magnetic filed unit 102is tuned to a frequency suitable to image a particular object orsubject.

The radio frequency magnetic field unit 102 is not limited to aparticular shape or volume. Preferably, the shape and volume of theradio frequency magnetic field unit 102 approximate the shape and volumeof the object or subject to be imaged. In one embodiment, the radiofrequency magnetic field unit 102 has a substantially cylindrical shape,including a diameter and a length sufficient to receive a human head. Inan alternative embodiment, the radio frequency magnetic field unit 102has a substantially cylindrical shape that includes a longitudinal axis116 and a surface 118 that is substantially parallel to the longitudinalaxis 116. The surface 118 need not be continuous. The current elements110-115 are arranged substantially parallel to the longitudinal axis.

The first aperture 104 provides a port for introducing an object orsubject into the radio frequency magnetic field unit 102. For example, ahuman head (not shown) can be introduced into the radio frequencymagnetic field unit 102 at the first aperture 104. The head ispreferably oriented within the radio frequency magnetic field unit 102such that the eyes are directed toward the second aperture 106. Withthis orientation, the subject avoids the claustrophobic effects oftenexperienced by subjects introduced into a radio frequency magnetic fieldunit that lacks a second aperture that is substantially unobstructed.The first aperture 104 is not limited to a particular alignment withrespect to the radio frequency magnetic field unit 102. In oneembodiment, the first aperture 104 has a center of mass point 120 thatis substantially aligned with the longitudinal axis 116. Such analignment permits easy introduction of the subject into the radiofrequency magnetic field unit 102. The first aperture 104 is formed atan end of the radio frequency magnetic field unit 102. An end of theradio frequency magnetic field unit 102 is located at an end of thecurrent elements 110-115.

The first aperture 104 is preferably contiguous to the second aperture106. A contiguous second aperture 106 permits relatively easyintroduction of a subject into the radio frequency magnetic field unit102 and reduces the likelihood that the subject will experienceclaustrophobic effects during imaging by providing a contiguous openspace that includes the first aperture 104 and the second aperture 106.The second aperture 106 also allows a subject to see outside the radiofrequency magnetic field unit 102 and allows a physician or technicianaccess to the eyes, nose and mouth of the subject.

The second aperture 106 comprises an area 122 including an unobstructedarea 124 and a potentially obstructed area. An area is unobstructed, ifthe area is substantially transparent. An area is obstructed, if thearea is not substantially transparent. Preferably, the area 122 does notinclude an obstructed area. The area 122 is not limited to a particularsize.

The second aperture 106 has a center of mass point 130 (not drawn toscale) and a first aperture axis 132. In one embodiment, the firstaperture axis 132 passes through the center of mass point 130,intersects the longitudinal access 116 and is substantiallyperpendicular to the longitudinal access 116. In one embodiment, thesecond aperture 106 subtends an arc 134 having an arc length 136 ofbetween about 0° and about 90° as traced out by the first aperture axis132 rotating about the longitudinal axis 118. The second aperture 106subtending an arc 134 having an arc length 136 of greater than 0° andabout 90° reduces claustrophobic effects in a human subject. However, anarc length 136 of greater than about 90° increases the difficulty ingenerating the desired magnetic field 108.

The second aperture 106 permits the manufacture of a radio frequencymagnetic field unit 102 that closely fits the head of a human subjecthaving a large nose. A radio frequency magnetic field unit that lacksthe second aperture 106 must be sized to accommodate the large nose of asubject and therefore cannot be designed to closely fit the head of ahuman subject having a large nose. Since a close fitting radio frequencymagnetic field unit produces higher quality images than a larger looselyfitting radio frequency magnetic field unit, the radio frequencymagnetic field unit 102 including the second aperture 106 produceshigher quality images than a radio frequency magnetic field unit thatlacks the second aperture 106.

An imaging unit 139 can be mounted on the radio frequency magnetic fieldunit 102 to provide a communication link to the second aperture 106. Theimaging unit 139 is located with respect to the second aperture 106 suchthat the imaging unit 139 provides a communication link to a subjectwhose head is positioned in the radio frequency magnetic field unit 102.The imaging unit 139 is not limited to a particular type of imagingunit. In one embodiment, the imaging unit 139 comprises a mirror. In analternative embodiment, the imaging unit 139 comprises a prism. In stillanother alternative embodiment, the imaging unit 139 comprises aprojection system.

In some embodiments, one or more apertures 144 and 145 are formed on aside of the radio frequency magnetic field unit 102 to permit access toa subject's ears. These apertures can be formed by removing a currentelement from a radio frequency magnetic field unit. In otherembodiments, an auditory communication device 146 is attached to one ormore of the one or more apertures 144 and 145 to communicate with asubject or provide auditory protection for the subject. Thecommunication device 146 is preferably capable of providing active orpassive auditory protection.

A radio frequency magnetic field unit lacking a second aperture can betransformed into the radio frequency magnetic field unit 102 thatincludes the second aperture 106. In one embodiment of a method totransform a radio frequency magnetic field unit lacking a secondaperture into the radio frequency magnetic field unit 102 that includesthe second aperture 106, one current element is removed from the radiofrequency magnetic field unit lacking a second aperture to form theradio frequency magnetic field unit 102 that includes the secondaperture 106. Removing one current element from a radio frequencymagnetic field unit lacking a second aperture creates a void in theradio frequency magnetic field unit lacking a second aperture. This voidprovides an area in which to form the second aperture 106. Afterremoving a current element from the radio frequency magnetic field unitlacking a second aperture, currents to produce the desired magneticfield 108 are calculated for the remaining current elements. In analternative embodiment, two or more adjacent current elements areremoved from a radio frequency magnetic field unit lacking a secondaperture to form the radio frequency magnetic field unit 102 thatincludes the second aperture 106. Removing two or more adjacent currentelements from an the radio frequency magnetic field unit lacking asecond aperture creates a void in the radio frequency magnetic fieldunit lacking a second aperture. This void provides an area in which toform the second aperture 106 of the radio frequency magnetic field unit102. After removing two or more current elements from the radiofrequency magnetic field unit lacking a second aperture, currents toproduce the desired magnetic field 108 are calculated for the remainingelectronic circuits.

FIG. 1B is an illustration of some embodiments of an imaging unit 140including a radio frequency magnetic field unit 102 according to theteachings of the present invention. The imaging unit 140 includes astatic field magnetic field unit 142 and the radio frequency magneticfield unit 102 located within the static field magnetic field unit 142.Preferably, the static field magnetic field unit 142 produces a magneticfield having a high magnetic field strength. A high magnetic fieldstrength enables the production of high resolution images by the imagingunit 140. However, the radio frequency magnetic field unit 102 is notlimited to use in connection with a particular static magnetic field ora static field magnetic field unit that produces a particular magneticfield strength. The radio frequency magnetic field unit 102 is suitablefor use in connection with any static field magnet used in connectionwith an imaging unit.

FIG. 2A is an illustration of some embodiments of an apparatus 200comprising a radio frequency magnetic field unit 202 according to theteachings of the present invention. The radio frequency magnetic fieldunit 202 includes a pair of end apertures 204 and 205, a first sideaperture 206 and a second side aperture 208.

The radio frequency magnetic field unit 202 generates a desired magneticfield 210. The desired magnetic field 210 is not limited to a magneticfield having a particular magnitude and direction. Preferably, thedesired magnetic field 210 has a magnitude and direction suitable foruse in imaging an object, such as a human body, in an imaging system,such as but not limited to a magnetic resonance imaging system, afunctional magnetic resonance imaging system or an electron spinresonance system.

The radio frequency magnetic field unit 202 is not limited to aparticular type of radio frequency magnetic field unit. In oneembodiment, the radio frequency magnetic field unit 202 is a TEM cavityresonator. A TEM cavity resonator includes one or more current elementshaving controllable elements that are varied to tune the transmissionline resonator. In one embodiment, the radio frequency magnetic fieldunit 202 comprises a first group of current elements 212 and a secondgroup of current elements 214. In one embodiment, the first group ofcurrent elements 212 include at least one current element, such ascurrent elements 216-218, and the second group of current elements 214include at least one current element, such as current elements 220-222.The first group of current elements 212 and the second group of currentelements 214 are preferably arranged such that none of the currentelements 216-218 or the current elements 220-222 obstruct the first sideaperture 206 or the second side aperture 208. In one embodiment, thefirst group of current elements 212 are separated from the second groupof current elements 214 by a separation distance 228 of between about 15centimeters and about 30 centimeters which is the area available to formthe first side aperture 206 and the second side aperture 208. Aseparation distance of less than about 15 centimeters is insufficient topermit extremities, such as arms or legs, or excess body mass, of asubject to fit into the first side aperture 206 and the second sideaperture 208. A separation distance 228 of greater than about 30centimeters results in the radio frequency magnetic field unit 202having a volume significantly greater than necessary to receive a humanbody. When used in connection with an imaging system (shown in FIG. 2B),the magnetic filed unit 202 is tuned to a frequency suitable to image aparticular object or subject.

A tunable TEM resonator according to the invention has a cavity and aset of transmission line segments which provide a high frequencymagnetic field in the cavity. Circuitry including the distributedimpedance of all the segments together determines the field frequency.

A preferred form of segment is a length of coaxial transmission line,wherein the center conductor's length is interrupted intermediately, sothat the circuitry, of which it forms part, incorporates it as ahalf-wave resonator balanced with respect to a virtual ground plane ofthe cavity.

The first side aperture 206 and the second side aperture 208 permit theextremities or excess body mass of a subject (not shown) to bepositioned outside the radio frequency magnetic field unit 202 when thesubject is located inside the radio frequency magnetic field unit 202.The first side aperture 206 and the second side aperture 208 aresubstantially parallel to the first group of current elements 212 andthe second group of current elements 214. The first side aperture 206and the second side aperture 208 are preferably free of physicalobstructions. A physical obstruction is a structure that prevents theextremities or excess body mass of a subject from extending into andthrough the first side aperture 206 or the second side aperture 208. Thefirst side aperture 206 and the second side aperture 208 also permit theradio frequency magnetic field unit 202 to receive subjects larger thanan inside diameter 230 of the radio frequency magnetic field unit 202without increasing the inside diameter 230 of the radio frequencymagnetic field unit 202. For many subjects, the first side aperture 206and the second side aperture 208, by allowing extremities or excess bodymass to extend outside the radio frequency magnetic field unit 202,increase the subject's comfort when positioned inside the radiofrequency magnetic field unit 202. A comfortable subject tends to moveless during imaging, and therefore fewer imaging retakes are requiredand higher quality images are obtained when the subject is imaged. Inaddition, the smaller, closer coil improves image quality significantlyfor body coils. The smaller sized coil can be made to resonateefficiently at the high frequencies required for high field strengthimaging.

The radio frequency magnetic field unit 202 is not limited to aparticular shape or volume. Preferably, the shape and volume of theradio frequency magnetic field unit 202 approximate the shape and volumeof the object or subject to be imaged. For example, a substantiallycylindrical radio frequency magnetic field unit having a length 232 ofabout 100 centimeters and the diameter 230 of about 60 centimeters has ashape that approximates the shape of a human body. In one embodiment,the radio frequency magnetic field unit 202 has a substantiallycylindrical shape, including a diameter and a length sufficient toreceive an adult human body.

In an alternative embodiment, the radio frequency magnetic field unit202 has a substantially cylindrical shape that includes a longitudinalaxis 234 and surfaces 236 and 238 that are preferably curved andsubstantially parallel to the longitudinal axis 234. The surfaces 234and 236 need not be continuous. The first group of current elements 212including the at least three current elements 216-218 and the secondgroup of current elements 214 including the at least three currentelements 220-222 are arranged to “enclose” a substantially cylindricalvolume.

The end aperture 204 provides a port for introducing an object orsubject into the radio frequency magnetic field unit 202. For example, ahuman body (not shown) can be introduced into the radio frequencymagnetic field unit 202 at the end aperture 204. The end aperture 204 isnot limited to a particular alignment with respect to the radiofrequency magnetic field unit 202. In one embodiment, the end aperture204 includes a center of mass point 240 that is substantially alignedwith the longitudinal axis 234.

In one embodiment, the first side aperture 206 and the second sideaperture 208 are contiguous to end aperture 204. A contiguousrelationship between the first side aperture 206, the second sideaperture 208 and the end aperture 204 permits easy introduction of asubject into the radio frequency magnetic field unit 202. The first sideaperture 206 has a width or separation distance 228 and the second sideaperture 208 has a width or separation distance 244. The width orseparation distance 228 is preferably about equal to the width orseparation distance 244.

In some embodiments, the radio frequency magnetic field unit 202includes a top-half 247 and a bottom-half 248, the top-half 247 capableof being mechanically attached and detached to the bottom-half 248 atthe first side aperture 206 or the second side aperture 208. In oneembodiment, an attachment device 249, such as a hinge or flexiblebracket, attaches the top-half 247 to the bottom half 248.

A radio frequency magnetic field unit lacking a first side aperture anda second side aperture can be transformed into the radio frequencymagnetic field unit 202 including the first side aperture 206 and thesecond side aperture 208. In one embodiment of a method to transform aradio frequency magnetic field unit lacking a first side aperture and asecond side aperture into a radio frequency magnetic field unit 202 thatincludes the first side aperture 206 and the second side aperture 208,two non-adjacent current elements are removed from the radio frequencymagnetic field unit lacking a first side aperture and a second sideaperture. Preferably, the two non-adjacent current elements are locatedopposite from one another. Removing two non-adjacent current elementsfrom the radio frequency magnetic field unit that lacks a first sideaperture and a second side aperture creates two voids in the radiofrequency magnetic field unit. These voids provide areas in which toform the first side aperture 206 and the second side aperture 208. Afterremoving two non-adjacent current elements from the radio frequencymagnetic field unit lacking a first side aperture and a second sideaperture, currents to produce the desired magnetic field 210 arecalculated for the remaining current elements.

FIG. 2B is an illustration of some embodiments of an imaging unit 250including the radio frequency magnetic field unit 202 according to theteachings of the present invention. The imaging unit 250 includes astatic-field magnetic field unit 252 and the radio frequency magneticfield unit 202 located within the static-field magnetic field unit 252.Preferably, the static-field magnetic field unit 252 produces a magneticfield having a high magnetic field strength. A high magnetic fieldstrength enables the production of high resolution images by the imagingunit 250. However, the radio frequency magnetic field unit 202 is notlimited to use in connection with a static-field magnetic field unit ora static-field magnetic field unit that produces a particular magneticfield strength. The radio frequency magnetic field unit 202 is suitablefor use in connection with any static-field magnetic field unit used inconnection with an imaging unit.

FIG. 2C is an illustration of some embodiments of the apparatusincluding the radio frequency magnetic field unit 202 shown in FIG. 2Aconfigured for use in a clinical setting. As can be seen in FIG. 2C, asubject easily and comfortable fits into a close fitting radio frequencymagnetic field unit 202 which makes radio frequency magnetic field unit202 particularly well suited for use in heart, lung and breast imagingapplications.

Although the embodiments described above were directed to radiofrequency magnetic field units for use in connection with imaging ahuman head and body, the radio frequency magnetic field units 102 and202 are not limited to use in connection with imaging a human head andbody. The radio frequency magnetic field units 102 and 202 are suitablefor use in connection with imaging a wide range of subjects includingbut not limited to human extremities, such as arms, legs, joints, handsand feet, non-human subjects, such as dogs, cats, mice, rats, horses,and primates and the extremities of those non-human subjects.

FIG. 3A-3D are illustrations of some embodiments of the structure of avolume coil 300 according to the teachings of the present invention. Thevolume coil 300 includes a cavity wall 301, which is not shaded so thatthe underlying structure of the volume coil 300 can be seen. The volumecoil 300 shown in FIG. 3A-3D includes current elements 302-308. Thevolume coil 300 shown in FIG. 3A includes a radio frequency conductivefront end ring 310 and a radio frequency conductive backplane 312. Theradio frequency conductive front end ring 310 and the radio frequencyconductive backplane 312 are coupled to the current elements 302-308.

The volume coil 300 shown in FIG. 3B includes a radio frequencyconductive front end ring 314 having a gap 316 and a radio frequencyconductive backplane 318 truncated to the current elements 302 and 308.The radio frequency conductive front end ring 314 and the radiofrequency conductive backplane 318 are coupled to the current elements302-308.

The volume coil 300 shown in FIG. 3C includes a radio frequencyconductive front end ring 310 and a radio frequency conductive back endring 319. The radio frequency conductive front end ring 310 and theradio frequency conductive back end ring 319 are coupled to the currentelements 302-308.

The volume coil 300 shown in FIG. 3D includes a radio frequencyconductive front end ring 314 having a gap 316 and a radio frequencyconductive back end ring 320 having a gap 322. The radio frequencyconductive front end ring 314 and the radio frequency conductive backring 320 are coupled to the current elements 302-308.

As can be seen in FIGS. 3A-3D, the volume coil 300 includes an aperture324 formed between the current elements 302 and 308. Also, as can beseen in FIG. 3A-3D, the aperture 324 is formed by removing a currentelement 326 (shown by a dashed line) from a regular or symmetricalarrangement of current elements that includes current elements 302-308and current element 326 (shown by a dashed line). In an alternativeembodiment, the current element 326 is removed from the top 328 of thevolume coil 300. In another alternative embodiment, the current element326 is displaced (rather than removed) to form an the aperture 324.

Each of the end rings 310, 314, 319 and 320 comprise an open end 330 ofthe volume coil 300 and each of the backplanes 312 and 318 comprise aclosed end 332 of the volume coil 300.

The volume coil 300 includes an impedance. In one embodiment, anadjustable impedance is included in each of the current elements302-308. The adjustable impedance, in one embodiment, is a capacitance.The adjustable impedance, in an alternative embodiment, is aninductance.

In one embodiment, the cavity wall 301 comprises return elements of thecurrent elements 302-308. In an alternative embodiment, the cavity wall301 comprises a slotted shield. As can be seen in FIG. 3A-3D, the cavitywall 301 includes an aperture in line with the missing or displacedcurrent element 326.

The volume coil 300 is suitable for use in imaging a wide range ofobjects and subjects including but not limited to heads, ankles, feet,and other extremities.

Each of the radio frequency magnetic field units 102 and 202 and thevolume coils 300 described above is suitable for use as a double tunedcoil, a multiply tuned coil, a circularly polarized coil, a coil doublytuned by the Vaughan method and an actively detuned coil. A double tunedcoil is driven at two frequencies. A multiply tuned coil is driven atmultiple frequencies. A circularly polarized coil is driven to impart acircularly polarized radio frequency magnetic field. The Vaughan methodof doubly detuning a coil is described in U.S. Pat. No. 5,557,247 titled“High Frequency Volume coils for Nuclear magnetic ResonanceApplications” which is hereby incorporated herein by reference. Each ofthe radio frequency magnetic field units 102 and 202 and the volume coil300 are capable of being actively detuned/retuned for use with a localreceiving coil by adjusting the current elements included in the coil.The current elements are adjusted by changing the impedance of thecurrent elements.

FIG. 4 is an illustration of one embodiment of a current element 400suitable for use in connection with the radio frequency magnetic fieldunits of the present invention. The current element 400 includes ashield or cavity wall section 402 resonant with a conductor 404. Thecavity wall section 402 is formed from a conductive material and theconductor 404 is formed from a conductive material. In one embodiment,the cavity wall section 402 is formed from a conductive mesh. Aplurality of current elements 400 can be arranged to form an“enclosure.” In one embodiment, a plurality of current elements 400 arearranged to form a cylindrical enclosure (not shown). In a cylindricalenclosure, the shield or cavity wall section 402 is oriented to theoutside of the enclosure and the conductor 404 is oriented to the insideof the enclosure. Current elements, such as current element 400, arefurther described in U.S. Pat. No. 5,557,247 which is herebyincorporated by reference herein.

Transmission line theory was used to describe the tuned TEM resonator asa transmission line tuned coaxial cavity resonator. Alternatively, theTEM resonator can be approximated as a balanced comb-line, band-passfilter using a lumped element circuit of FIG. 6A. The lumped elements inthis circuit approximate the distributed element coefficients of thetransmission line circuit. Analysis of this lumped element filtercircuit model adhering to methods in the literature for “bird-cage”resonators gives inaccurate results. A more accurate approach considersthe lumped element filter's distributed stripline analogue in FIG. 6B.This network is a quarter wave (as in FIGS. 5A and 5C) comb-line filterinterfaced with its mirrored image at the virtual ground plane ofsymmetry indicated by the dotted line. Each coaxial element, due to itssplit central conductor, therefore is a resonant half wave line(mirrored quarter wave pair, as in FIGS. 5B and 5D wave pair) whosebisected center conductor 11 is grounded at both ends to a cavity. Theelements 9 are coupled via the TEM slow wave propagation h the cavity.The performance characteristics of this distributed structure arecalculated from TEM assumptions.

Because the TEM coil has no endring currents (as does the birdcage),sections of the TEM coil can be removed entirely to provide maximumaccess with minimal impact to the compensated RF field of the inventionvolume coil. Because the TEM coil return current is parallel to the coilrungs, the return paths can be discretized to narrow, unobtrusiveconductors such as 1 cm strips of transparent screen. The integrity ofthe TEM cavity is thus approximately maintained while providing throughthe rung access, in addition to the entirely unobstructed accessprovided by removal of both an element and its corresponding return pathon the cavity.

Although specific embodiments have been described and illustratedherein, it will be appreciated by those skilled in the art, having thebenefit of the present disclosure, that any arrangement which isintended to achieve the same purpose may be substituted for a specificembodiment shown. This application is intended to cover any adaptationsor variations of the present invention. Therefore, it is intended thatthis invention be limited only by the claims and the equivalentsthereof.

1. An apparatus comprising: a volume coil including a plurality ofcurrent elements, the volume coil having an aperture formed by removalor displacement of one or more current elements from a regular orsymmetric pattern or arrangement of current elements. 2-116. (canceled)